blending: Document available blending modes and how to apply them to clips

We exposed the "operator" property of the compositor in GStreamer, through
which we can change a clip's blending mode. We also implemented a selector
for a clip's blending mode in Pitivi for users to interact with.

This change documents the location of the blending mode selector so that
users can easily find it. It also documents what each of the blending
modes do, which is valuable because the names may not be intuitive to users.
We decided to place the change in the "Effects and Transitions" section of
documentation due to similar methods of application and somewhat familiar
functionality.

Related to, but does not close:
GNOME/pitivi#2313
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